1. Field of the Invention
The present invention relates to measurement devices and, more particularly, to a contact-free, optical measurement systems for determining the precision with which a surface spins about an axis. In a preferred embodiment, the spinning surface is associated with, and related to, the manufacture of integrated circuit chips.
2. History of the Prior Art
The use of spinning surfaces such as mandrels, spindles and the like dates back into technological antiquity. Early craftsman, for example, formed clay pots by using a spinning base upon which the clay to be molded was positioned. The technique is used even today. The spinning of the base permits not only uniform application of molding forces around the object being formed but also the incorporation of certain centrifugal forces thereto. It is the latter aspect of spinning mandrels that has found a niche in high tech fabrication techniques, including the sophisticated, and technically challenging, semi-conductor manufacture.
In the manufacture of semi-conductor devices, such as integrated circuits, a plurality of devices are formed on a single wafer of silicon material. Each wafer is typically circular and on the order of 6 inches in diameter. The wafers are put through a number of sequential processing steps, including washing, spinning and coating them with photo-resists, exposing them to the optical patterns formed on photo masks, exposing them to both liquid and gaseous treating environments, and passing them through high temperature baking operations.
The processing of a silicon wafer containing a plurality of semi-conductor devices requires a high degree of cleanliness and precision in the environment in order to produce acceptable devices. The ability of a semi conductor device to perform satisfactorily from both electrical and mechanical standpoints depends upon the nature and quality of the materials forming the various layers of the device and the precision of various manufacturing steps. Errors in processing and/or the introduction of any foreign matter into the environment where the wafers are being handled results in a decrease in the "yield" of the wafer. The yield is the number of devices that can pass the required electrical tests of the device after the processing has been completed. This is usually expressed as a fraction of the total number of devices processed on the wafer that did pass the required tests Thus, the higher the precision of the processing steps used in manufacturing the semi-conductor devices, the greater the yield and hence the greater the financial return to the manufacturer.
The semi-conductor wafers are generally handled between processing steps in inert plastic frames containing a plurality of vertically arranged dividers. Each divider defines a pocket to receive a silicon wafer and holds it securely in a vertical orientation while isolating it from adjacent wafers. The plastic frame and its dividers are together referred to as a wafer "cassette." A typical industrial cassette may hold on the order of 25 wafers for processing. The cassettes are physically moved by operators from one processing station to another wherein the cassettes are placed in indexing mechanisms that generally form a part of the processing machinery of each station. Each wafer is then automatically removed from the cassette for processing and returned by the indexing mechanism after the processing step of each station has been completed.
One step of the fabrication technique requires that the wafer be removed from the cassette and mounted onto a mandrel for rotation through a wide range of RPM's. These units are commonly referred to as "spinners." The rotation of the spinner generates centrifugal forces utilized to remove and/or control the thickness of various liquids deposited on the wafer surface. If the deposited liquid is a photo-resist, the spinner is generally part of a coating line If the deposited liquid is a developer, the spinner is part of a development track In the application of such liquids, the wafer may be spun at high RPMs in order to drive off the liquid by centrifugal force. In certain steps, however, more viscous fluids, such as photo-resist are deposited thereon. The wafer is rotated at a different speed for establishing a uniform spread and a controlled thickness of the fluid coating. For proper results, it is imperative that the precision of rotational spin and axle alignment be carefully maintained and monitored.
It is well known to measure rotational tolerance of spindles and the like with feeler gauges and other sensors that contact the spinning surface. The reliability and response time of such mechanical devices is not, however, always sufficient for the high degree of precision and/or speed necessary in semi-conductor fabrication. This is particularly true when the fabrication technique utilizes a high speed spinner, because the response time of a feeler gauge is oftentimes greater than the rate of rotation. Accurate measurement would not then be possible. For this reason, more precise measurement devices are needed in the semiconductor fabrication industry.
The design of precise measurement systems for a variety of industrial applications is well established. Many of these systems use optical arrays which may include lasers as the light source. One such system is shown in U.S. Pat. No. 4,995,726 which utilizes an optical array to detect the surface profile utilizing optical heterodyne interference. Yet another optical linear measurement device is seen in U.S. Pat. No. 4732486, wherein a semi-reflecting mirror reflects the light of a first laser beam path to a path diverting mirror to generate a second beam path the course of which is parallel to the first beam path. An object to be measured is positioned at right angles in relation to and within the two beam paths. Light sensors receive darkened light signals in the respective beam paths behind the object to be measured. By means of electronic evaluation, the inclination of the object to be measured can be determined and the exact diameter of the object can be calculated without being influenced by its inclination.
Sophisticated measurement systems, such as those described above, utilize not only lasers for emitting the light, but sophisticated electronic evaluation units in conjunction with control systems for rotating the mirrors being used therewith. While effective for high precision, contact free optical measurement, such systems do not afford a basic, inexpensive device for measuring the precision with which an object rotates. Indeed, the very genesis of the above-described laser mounted systems do not find their application in such areas. There thus remains a need in the semi-conductor manufacturing industry, and in quality control operations for spindles in general, for the determination of precision with which a spindle rotates, both with and without loads and through a wide range of RPM's.
The present invention provides such a measurement system by utilizing a light source, preferably provided by a laser, which light source is projected relative to a target surface in a position facilitating a visual inspection of rotational precision. The inspection, or measurement, may be performed by an operator without the aid of expensive and complex systems to determine the precision by which a reflective surface rotates there beneath.